The invention relates to a process for the preparation of alkali metal monohydridoboranates and -aluminates of the general formula: 
wherein
M=Li, Na, K, Rb or Cs and
E=B or Al, and
X1, X2, X3=in each case independently of one another, is
a 2 to 10 C atom secondary or tertiary alkyl group or
a phenyl group, which in its turn can be alkyl-substituted, or
an alkoxy group.
Alkali metal monohydridoboranates and -aluminates are to some extent known classes of compounds, the members of which have found uses as reagents in chemical synthesis, e.g. as reducing agents. Thus, for example, commercially available lithium tri-tert-butoxyaluminium hydride is employed for the chemoselective reduction of acid chlorides to aldehydes or for the stereoselective reduction of asymmetrically substituted or cyclic ketones to alcohols (P. Galatsis, xe2x80x9cLithium-tri-tert-butoxyaluminiumhydride in L. A. Paquette, Encyclopaedia of Reagents for Organic Synthesis, J. Wiley and Sons, Chichester 1995, p. 3168-3172).
In a similar manner, trialkyl borohydrides also serve as diversely usable reducing agents in organic synthesis. In general, their stereoselectivity increases with the steric bulkiness of the alkyl substituents. (H. C. Brown, S. Krishnamurthy, J. L. Hubbard, J. Am. Chem. Soc. 1978, 100, 3343; R. Kxc3x6ster, xe2x80x9cAnionische Organobor-Wasserstoff-Verbindungen [Anionic Organoboron-Hydrogen Compounds]xe2x80x9d in: Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry] 13/3b, p. 798-813, G. Thieme Verlag, Stuttgart, 1983; J. L. Hubbard, xe2x80x9cLithium tri-s-butylborohydridexe2x80x9d in: L. A. Paquette, Encyclopaedia of Reagents for Organic Synthesis, J. Wiley and Sons, Chichester 1995, 3172-3176; J. D. Odom, in: Comprehensive Organometallic Chemistry, G. Wilkinson (ed.), Pergamon Press 1982, vol. 1, p. 297).
Some representatives of the alkali metal monohydridoboranates and -aluminates can be obtained by addition of alkali metal hydride (MH) to the EX1X2X3 compound. This applies e.g. to the preparation of Li[HBEt3] in accordance with: 
where Et=ethyl.
Without a solvent or with a hydrocarbon as the solvent, the reaction at 200xc2x0 C. takes approx. 4 h, and with Et2O as the solvent the reaction on heating under reflux takes approx. 24 h (R. Kxc3x6ster, xe2x80x9cAnionische Organobor-Wasserstoff-Verbindungenxe2x80x9d in: Houben-Weyl, Methoden der organischen Chemie 13/3b, p. 798-813, G. Thieme Verlag, Stuttgart, 1983). In THF solution, the addition can be effected within one day at room temperature.
The induction time of the reaction presents problems. This method also fails if boranes with bulky substituents are employed. For example, the reaction of B(sBu)3 with alkali metal hydrides in boiling THF gives a conversion of only 10% after 24 h, and for this reason this reaction is unsuitable for a commercial synthesis. (H. C. Brown, S. Krishnamurthy, J. L. Hubbard, J. Am. Chem. Soc. 1978, 100, 3343; R. Kxc3x6ster, xe2x80x9cAnionische Organobor-Wasserstoff-Verbindungenxe2x80x9d in: Houben-Weyl, Methoden der organischen Chemie 13/3b, p. 798-813, G. Thieme Verlag, Stuttgart, 1983; J. L. Hubbard, xe2x80x9cLithium tri-s-butylborohydridexe2x80x9d in: L. A. Paquette, Encyclopaedia of Reagents for Organic Synthesis, J. Wiley and Sons, Chichester 1995, 3172-3176; J. D. Odom, in: Comprehensive Organometallic Chemistry, G. Wilkinson (ed.), Pergamon Press 1982, vol. 1, p. 297).
Boranes with even bulkier substituents are inert with respect to xe2x80x9cnormalxe2x80x9d, i.e. commercially obtainable, NaH and LiH in boiling THF. This does not apply to the highly reactive form of the binary hydrides, such as are prepared e.g. by decomposition of alkyllithium solutions under a hydrogen atmosphere. (R. Pi, T. Friedl and P. v. R. Schleyer, J. Org. Chem. 1987, 52, 4299-4304). Because the active metal hydride first has to be prepared from expensive organolithium solutions, this process is of little commercial interest.
Another variant of preparing active metal hydride comprises preparing the metal, preferably in finely divided form, in the presence of a trisubstituted boron compound, so that the hydride MH formed in situ can add on to the boron compound, immediately, to form a borohydride of the formula M[R1R2R3B]H. Disadvantages in this case are that the metal must be present in the form of a highly reactive powder which is difficult to handle, and a catalyst combination in the form of a transition metal salt (e.g. FeCl3) and/or polyaromatics (e.g. phenanthrene) must be employed to achieve reasonable reaction temperatures and times (U.S. Pat. No. 5,886,229). The product solutions accordingly are contaminated and are discoloured by the transition metal content.
Trialkoxy-element hydrides with bulky substituents also do not react or react only extremely slowly with MH. For example, the preparation of lithium tri-tert-butoxyaluminium hydride (LTTBA) in accordance with: 
is unknown (see also Comparative Example A).
Since the direct preparation is not possible, a number of process alternatives have been developed. Thus, LTTBA is prepared by alcoholysis of lithium aluminium hydride in accordance with: 
The high preparation costs are a disadvantage, since relatively expensive hydride hydrogen in the LiAlH4 is destroyed by the alcoholysis.
Trialkyl borohydrides with bulky organic radicals are prepared by one of the following general processes: 
Disadvantageous with the process according to (4) are the use of expensive LiAlH(OMe)3, which is not commercially obtainable, and above all the fact that large amounts of insoluble aluminium methylate are obtained, which makes preparation of the trialkyl borohydride in a pure form extremely difficult. Similar circumstances apply to process (5), and in addition there are high costs for the donor, such as e.g. 1,4-diazabicyclo[2,2,2]octane (DABCO).
Process (6) has the disadvantage that expensive t-butyllithium is used as the LiH source, a gaseous by-product being formed. Furthermore, the reaction must be carried out at very low temperatures, which is very unfavourable in energy terms.
All the processes (3)-(6) have the disadvantage that they are limited in practice to the preparation of the lithium derivative, since only the corresponding lithium raw materials (and not the Na or K compounds) are commercially obtainable. The addition of higher alkali metal hydrides (NaH, KH, RbH, CsH) to a starting compound EX1X2X3 indeed proceeds substantially faster than in the case of LiH, but in these cases also the rate of reaction decreases sharply with increasing volume of the substituents X (see comparison example B).
The object of the invention is to overcome the disadvantages of the prior art and to provide a process for the rapid preparation of alkali metal monohydridoboranates and -aluminates of the general formula: 
at mild temperatures which starts from commercially available alkali metal hydride, allows a reaction procedure without increased pressure and avoids the formation of insoluble by-products.
The object is achieved by the process described in claim 1. Claims 2 to 11 develop the process described. Claim 12 describes preferred process products.
It has been found that the addition described above of alkali metal hydride (MH) on to an EX1X2X3 compound is significantly accelerated by a catalyst: 
where
M=Li, Na, K, Rb or Cs and
E=B or Al and
X1, X2, X3, in each case independently of one another, =
a secondary or tertiary alkyl group consisting of 2 to 10 C atoms or
a phenyl group, which in its turn can be alkyl-substituted, or
an alkoxy group.
Any boron-containing compound which contains the structural unit BH3 and which itself or the reaction product of which with MH is capable of acting as a hydride transfer agent can be employed as the catalyst.
X1, X2 and X3, in each case independently of one another, can preferably be iso-propyl or sec-butyl or tert-butyl or tert-amyl or siamyl (sec-2-methyl-butyl) or a phenyl group, which in its turn can be alkyl-substituted, or the following alkoxy group: 
where Rxe2x80x21, Rxe2x80x22, Rxe2x80x23, independently of one another, =alkyl having 1 to 10 C atoms.
In the case of liquid compounds EX1X2X3, the reaction can in principle be carried out without a solvent; however, working in a solvent is preferred, or unavoidable in cases where EX1X2X3 is not liquid. Aprotic organic compounds, such as e.g. hydrocarbons and/or ethers, are used as the solvent. The reaction proceeds faster in polar solvents than in non-polar hydrocarbons.
In principle, the sequence of the addition of the individual reaction partners plays no role. Preferably, the total amount of MH is suspended in the solvent, the catalyst is added and the compound EX1X2X3 is metered in as a function of the rate of reaction.
Borane complexes H3B.D with a donor compound D are particularly suitable as the catalyst. Amines, preferably secondary amines, can be employed e.g. as the donor compound.
Aminoborohydrides of the general formula:
M[R2R1NBH3]nxe2x80x83xe2x80x83(D)
where M=Li, Na, K, Rb, Cs or Mg halogen if n=1, or M=Mg if n=2 and
R1, R2=independently of one another H, alkyl or aryl,
wherein R1 and R2 can be bonded to one another via a ring closure, or
M[R2R1NBH2NR3R4BH3]nxe2x80x83xe2x80x83(E)
where M=Li, Na, K, Rb, Cs or Mg halogen if n=1 or M=Mg if n=2 and
R1, R2, R3, R4=independently of one another H, alkyl or
aryl, wherein R1, R2, R3 and/or R4 can be bonded to one another via a ring closure,
can also be employed as the catalyst.
The catalyst is in general employed in an amount of 0.1 to 20 mol %, and in individual cases it may prove favourable to choose an even higher dosage. The catalyst is preferably employed in an amount of 0.5 to 5 mol %.
The temperature of the reaction is 0 to 150xc2x0 C., depending on the substrate and solvent.
The process according to the invention has the advantage that alkali metal monohydridoboranates and -aluminates can be prepared from commercial alkali metal hydrides in a simple manner, rapidly and without the use of pressure. In particular, the alkali metal monohydridoboranates and -aluminates with sterically voluminous substituents are accessible to synthesis in this manner.
An alkali metal tri-tert-butoxyaluminium hydride or an alkali metal tri-sec-butyl borohydride or alkali metal tri-(sec-2-methyl-butyl)-borohydride can preferably be obtained as compound (A) by the process according to the invention.
The alkali metal monohydridoboranates and -aluminates prepared by the process according to the invention are used as reducing agents in organic synthesis.
The invention is explained in more detail in the following with the aid of examples.